1. Field of the Invention
The present invention is generally concerned with machines for carrying out the machining or trimming that is normally required to match the contour of an eyeglass lens to that of the rim or surround of the eyeglass chassis to which it is to be fitted.
2. Description of the Prior Art
It is more particularly concerned with machines of this kind which include at least one cutting tool, such as a grinding wheel or a milling cutter, rotatably mounted on a chassis, a carriage mounted to pivot on the chassis about an axis parallel to the rotation axis of the cutting tool, a support spindle mounted on the carriage to rotate about an axis parallel to the rotation axis of the cutting tool and adapted to receive the lens to be machined axially and in vertical alignment with the cutting tool, a template spaced from the lens and a feeler, usually called a vernier head, carried by the chassis in vertical alignment with the template to support the carriage through the intermediary of the template and so to halt at a set point the relative interengagement of the glass and the cutting tool.
The template can have the exact contour of the rim or surround of the eyeglass chassiss to which the lens is to be fitted, in which case the feeler can be fixed.
Alternatively, the template can have a circular contour in which case the associated feeler is mobile on the chassis under the control of control means which output shape data transmitted to them by a contour reading device operating on the rim or surround of the eyeglass chassis to which the lens is to be fitted, imposing an instantaneous set point of the feeler for a given angular orientation of the support spindle carrying the lens to be machined.
In all cases an appropriate bearing load must be provided between the lens and the cutting tool.
This bearing load is usually the result of the combined effect of the weight of the carriage and the action of ancillary means provided for this purpose.
For example, these ancillary means can include a counterweight mounted on the carriage with its transverse position relative to the carriage pivot axis adjustable.
Although adjusting the counterweight advantageously varies the bearing load that it produces, by variation of the lever arm, it has the drawback of being bulky and costly.
The ancillary means employed can instead include spring means.
At present these spring means, which are in the form of a prestressed spring, for example, are disposed between the carriage and the chassis and so are adapted to urge the carriage towards the feeler carried by the chassis.
However, the bearing load that they produce at the lens being machined is difficult to adjust and increases as the diameter of the lens is reduced.
For some applications at least, for example when trimming a thin or fragile lens, it is desirable to use a moderate bearing load which is kept constant, whereas in other applications, for example for trimming a standard lens, it is possible to use a higher initial bearing load and to accept some subsequent variation in the load.
A general object of the present invention is to provide an arrangement which meets this two-fold requirement in a very simple manner.